Field of the Invention
The present invention relates to an optical system and particularly to an optical system preferable as an imaging optical system used in image pickup apparatuses such as a monitoring camera, a digital camera, a video camera, and a broadcast camera.
Description of the Related Art
In recent years, there has been a demand that an imaging optical system used in image pickup apparatuses such as a monitoring camera should achieve a wide angle of view, a preferable optical performance in imaging in both daytime and nighttime, and other characteristics. Generally, the monitoring camera uses visible light for the imaging in daytime and near-infrared light for the imaging in nighttime. The near-infrared light has such a favorable characteristic that in, for example, dense fog in which the visibility is low, imaging with the near-infrared light is less affected by scattering than imaging with the visible light. Accordingly, there is a demand that the imaging optical system to be used in a monitoring camera should support aberration corrections over a wide wavelength range from a visible range to a near-infrared range.
Conventionally, there is known an imaging optical system preferable for a monitoring camera which supports aberration corrections over the wide wavelength range from the visible range to the near-infrared range. Japanese Patent Application Laid-Open No. 2004-354829 discloses a fixed focal lens including, in order from an object side to an image side, a front lens group with a negative refractive power, an aperture stop, and a rear lens group with a positive refractive power. Japanese Patent Application Laid-Open No. 2011-128273 discloses a medium-telephoto imaging lens including a first group with a positive or negative refractive power, a second group with a positive refractive power, and a third group with a positive or negative refractive power.
An objective lens for a microscope is also known as an optical system in which various aberrations are well corrected over the wide wavelength range from the visible range to the near-infrared range. Japanese Patent Application Laid-Open No. H04-26813 discloses an objective lens for a microscope which includes, in order from an object side to an image side, a first lens unit with a negative refractive power and a second lens unit with a positive refractive power.
In many cases, an imaging optical system for a monitoring camera uses near-infrared light for imaging in nighttime. However, in times close to the new moon when the amount of moonlight is extremely low or in the case where the moon is hidden behind clouds, sufficient amount of near-infrared light cannot be obtained in some cases. Meanwhile, after being excited by sunlight, hydroxide ions in the atmosphere emit light called nightglow (peak wavelength of 1.6 μm). Excellent imaging can be easily performed even without moonlight by using this light.
In order to obtain a wide angle of view and an excellent optical performance over the wide wavelength range from the visible range to the near-infrared range in the imaging optical system for a monitoring camera, it is important to appropriately set the configurations of lenses forming the imaging optical system. For example, in the imaging optical system including, in order from the object side to the image side, the first lens unit with the negative refractive power, the aperture stop, and the second lens unit with the positive refractive power, appropriately setting the materials of the lenses forming the second lens unit is important to correct a chromatic aberration over the wide wavelength range in an excellent manner.